Bugfix: List snapshots in backend at most once to resolve snapshot IDs

Many commands support specifying a list of snapshot IDs which are then used to
determine the snapshots to be processed by the command. To resolve snapshot IDs
or `latest`, and check that these exist, restic previously listed all snapshots
stored in the repository. Depending on the backend this could be a slow and/or
expensive operation.

Restic now lists the snapshots only once and remembers the result in order to
resolve all further snapshot IDs swiftly.

https://github.com/restic/restic/issues/3428
https://github.com/restic/restic/pull/3570
https://github.com/restic/restic/pull/3395
